1. Field of the Invention
The present invention relates to an elastomer and to a process for production thereof. More particularly, the present invention relates to an isocyanate-based (e.g. polyurethane, polyurea, polyisocyanurate, etc.) elastomer and to a process for production thereof.
2. Description of the Prior Art
Isocyanate-based elastomers are known in the art. Generally, those of skill in the art understand isocyanate-based elastomers to be polyurethanes, polyureas, polyisocyanurates and mixtures thereof.
It is also known in the art to produce isocyanate-based elastomers using various techniques. Indeed, one of the advantages of isocyanate-based elastomers compared to other elastomer systems is that polymerization can be controlled to a degree sufficient to enable molding of the elastomer while it is forming.
One of the conventional ways to produce a polyurethane elastomer is known as the "one-shot" technique. In this technique, the isocyanate, a suitable polyol, a catalyst and other additives are mixed together at once using, for example, an impingement mixer. Generally, if one were to produce a polyurea elastomer, the polyol would be replaced with a suitable polyamine. A polyisocyanurate elastomer may result from cyclotrimerization of the isocyanate component. Urethane-modified polyurea or polyisocyanurate elastomers are known in the art. In either scenario, the reactants would be intimately mixed very quickly using a suitable mixer.
Another technique for producing isocyanate-based elastomers is known as the "prepolymer" technique. In this technique, a prepolymer of polyol and isocyanate (in the case of a polyurethane) are reacted in an inert atmosphere to form a liquid polymer (i.e. a prepolymer) terminated with isocyanate groups. To produce the elastomer, the prepolymer is thoroughly mixed with a lower molecular weight polyol or other active hydrogen-containing compound.
Regardless of the technique used, it is known in the art to include a filler material in the reaction mixture. Conventionally, filler materials have been introduced into elastomers by loading the filler material into one or both of the liquid isocyanate and the liquid active hydrogen-containing compound (i.e. the polyol in the case of polyurethane, the polyamine in the case of polyurea, etc.).
The nature and relative amounts of filler materials used in the reaction mixture can vary, to a certain extent, depending on the desired physical properties of the elastomer product, and limitations imposed by mixing techniques, the stability of the system and equipment imposed limitations (e.g. due to the particle size of the filler material being incompatible with narrow passages, orifices and the like of the equipment).
Typically, when it is desired to load the elastomer with a filler material, there are limitations on the process resulting from the increase in the viscosity of the reaction mixture as polymerization proceeds. Additional limitations result from the difficulties encountered in achieving substantially complete wetting-out of filler materials in the case where all ingredients of the reaction mixture (including the filler material) are mixed in one step in a suitable mix head (i.e. the "one-shot" technique).
A particular difficulty is encountered in the situation where the nature and surface structure of the filler material renders it selectively compatible with some but not all of the ingredients in the reaction mixture. The result of this is that, regardless of whether adequate mixing of ingredients is achieved, there is an imbalance in the physical allocation of the filler material in the elastomer product with the filler material essentially agglomerating. This results in non-uniform physical properties in the product. A secondary effect is the relative (and localized) withdrawal of one or more components of the homogeneous, liquid reactant system which may cause catastrophic elastomer property alterations. While, equipment modifications are useful to mitigate one or more of these problems, there is still a need to facilitate addition of the filler material to the reaction mixture with a view to further mitigating or even eliminating one or more of the problems. Specifically, it is known to add one additional stream to the process with a view to separating the filler material from other components in the reaction system which cannot tolerate the filler material. For example, this may be done by having a dispersion of the filler material and the active hydrogen-containing compound (the majority or all) separated from the amine-based extenders and other components used in the reaction system.
When a filler material is added to any reaction mixture used to produce an isocyanate-based elastomer, it is desirable during the process to achieve both (i) uniform distribution of the filler material throughout the elastomer matrix, and (ii) intimate contact (both chemical and physical) between the filler material, and the isocyanate and active hydrogen-containing compound. The result of not achieving both (i) and (ii) above can cause uncontrolled physical property variations in the resulting elastomer product due to an uneven distribution of the filler material. This is particularly a problem in the case when the individual particles of filler material are not separated from each other and the resulting product contains lumps of either "dry" or "wetted" and agglomerated filler material particles. Moreover, in the prior art processes, as the loading of filler material has been increased, the surface quality of the resulting product has deteriorated. Specifically, since the filler material is "foreign" to the reactive system, it has a tendency to be "washed out" to the surface of the finished product.
Attempts have been made to overcome these problems by addition of the filler material to the reaction mixture in specially designed low pressure mixing heads. These mixing heads essentially endeavour to achieve both (i) and (ii) in a single step. While these mixing heads provide for adequate mixing of the filler material and the reaction mixture, it is not typically possible to obtain high loadings of filler material due to the fact that, at increased loadings of the filler material, the mixing heads do not provide uniform distribution of the filler material simultaneously with the required intimate mixing (at the molecular level) of the main chemical reactants. The reason for this is that as the polycondensation reaction proceeds, the viscosity of the reaction mixture increases resulting very rapidly in a reduction in the ability to achieve (i) and (ii) above. Practically, this translates into an inability to achieve filler loading levels (using filler materials having a relatively low specific gravity) of greater than about 17-30% by weight of the elastomer reaction mixture without adversely affecting the physical properties of the final elastomer product. Furthermore, if the efficiency of mixing the reactants and filler material is insufficient, a separation effect of the filler material (by particle size) is likely to occur resulting in an uneven particle size distribution in the cross-section of the elastomer mass.
Other attempts to overcome these difficulties include the use of high pressure mixheads. Specifically, the filler material stream (especially if the filler material is a similar chemical nature as the finished isocyanate-based elastomer) is separated from the active hydrogen-containing compound stream in order to overcome selective absorption of some chemical components into the filler material from the active hydrogen-containing compound stream. It has been reported that this technique may be used to achieve a filler material loading level up to 30% by weight of the final product using a dispersion of filler material (particle size up to about 200 .mu.m) having a relatively low specific gravity (e.g. recycled materials such as post-user and post-consumer goods).
Regardless of the mixing technique used, it is known generally in the art to produce isocyanate-based elastomers by either: (i) pouring the reaction system into an open mold (also referred to as "cast technology"); or (ii) injecting the reaction system into a closed mold (also referred to as "Reaction Injection Molding technology" (RIM) or "Reinforced Reaction Injection Molding technology" (RRIM)). These molding techniques combine the reactive properties of the mixed components with the mold conferred shape retention of the mold cavities used for finishing the chemical reactions.
Obvious flow-related limitations (e.g. mold-created back pressure, time/temperature viscosity function, etc.) exist in RIM technology, and part thickness and surface area are dependent on both the reactivity the components used in the system and the performance of the equipment. Further, it is believed that it may be necessary to modify at least a portion of the surface of the filler material to be able to incorporate even relatively minor loading (e.g. 15% by weight) thereof material without an adverse effect on the finished article. For example, it is known to treat the filler material to create further reactive sites on the surface thereof.
While use of cast technology can obviate or mitigate the flow-related problems associate with RIM technology, the process is slow relative to RIM or RRIM technology or may be inappropriate to use in certain applications. Moreover, for certain applications, it is known that the polymer structure of a product made using cast technology is inferior to a product made by the use of RIM or RRIM technology.
In light of these difficulties in the prior art, it would be advantageous to have a process for producing an isocyanate-based elastomer which is relatively simple, can utilize a large variety of filler materials and allows for the introduction of substantially large amounts of filler materials without the need for the prior art specialized mixing equipment. It would be especially advantageous if such a process (i) could be adapted to utilize filler materials based on recycled isocyanate-based polymers or elastomers or other post-consumer and post-user products (e.g. tires), and (ii) was not limited by the particle size of the filler material. It would also be desirable if the process were able to obviate or mitigate the disadvantages associated with both cast technology and reaction injection mold technology.